The present invention relates generally to liquid abrasive jet cutting devices and, more particularly, to a focusing tube for a liquid abrasive jet cutter for making non-perpendicular cuts in a workpiece.
It is known generally to use a high velocity liquid abrasive jet for making precise cuts in a variety of materials. Liquid abrasive jets are typically formed by producing a forceful stream of liquid at high pressure, aligning the liquid jet with a focusing tube, introducing fine abrasive particles into the liquid jet stream at an inlet end of the focusing tube, and accelerating the abrasive particles through the length of the focusing tube to form a stream of liquid and abrasive which is discharged at the outlet end of the tube onto a workpiece to be cut.
For the liquid abrasive jet to have sufficient cutting power, the stream of liquid and abrasive particles must be tightly concentrated in a coherent pattern as it strikes the workpiece. Maintenance of a coherent stream is essential if the liquid or abrasive jet is to have effective cutting power and to allow the jet to cut with a small kerf or hole, as is required for precision cutting operations.
The distance which the jet of liquid and abrasive must travel from the focusing tube tip to the workpiece, known as the stand-off distance, has a significant effect on the coherence of the jet pattern, since the jet rapidly disperses over distance. The liquid/abrasive jet also quickly loses energy as it moves away from the focusing tube, thereby resulting in a loss of effective cutting power. Therefore, it is desirable to place the outlet end of the focusing tube as close to the workpiece as possible, in order to reduce the stand-off distance.
For cuts that are to be made generally perpendicular to the surface of the workpiece, positioning of the focusing tube immediately adjacent the workpiece surface does not present any unusual problems. However, for cuts that are to be made at a non-perpendicular angle to the workpiece surface, minimizing the distance between the focusing tube tip and the surface to be cut can be difficult. Conventional focusing tubes are often formed in a cylindrical configuration, with a relatively thick cylinder wall that is squared at the tip of the tube so as to prevent chipping or other damage to the tip. Hashish et al U.S. Pat. Nos. 4,648,215 and 5,320,289 both disclose focusing tubes of this type, with the focusing tube disclosed therein being referenced as a "nozzle." Focusing tubes having an outlet end which is evenly tapered in a frusto-conical configuration are also known, as disclosed in Hashish et al U.S. Pat. No. 4,955,146. It is not desirable for the end to taper continuously down to form a knife edge at its extremity, however, as such a configuration would be particularly vulnerable to chipping or breakage in handling, while the thin knife edge would also quickly wear away as a result of the abrasive action of the jet. Consequently, conventional focusing tubes having a tapered outlet end typically include a squared-off portion at the extreme end to eliminate formation of a knife edge.
None of the above-described focusing tube configurations permit the tube to be positioned immediately adjacent the workpiece surface for a non-perpendicular cut. Thus, the liquid abrasive jet loses cutting power and produces a wider kerf than is desirable. A need, therefore, exists for a liquid abrasive jet focusing tube which minimizes the stand-off distance for non-perpendicular cuts.